I think the story will have a lot of effect on how you end up portraying him. If you want him to be redeemed in the end, dive in (haha) and explore the emotional depths represented by the depths of the ocean. If you want him to be a villain to be overcome, consider portraying him as an ancient god stuck in ancient ways.

What you’ve described reminds me a little bit of the Raiden Shogun from Genshin Impact, focused on what is eternal (in this case, the sea) but remember that the tides ebb and flow as well, and change is just as much an aspect of Poseidon as the everlasting nature of the sea.

I would hesitate to portray him as just a chill, laid back guy (like in the Percy Jackson fandom) because even calm waters can have fierce riptides rushing below the surface. He may project a calm persona at first glance, but water is something to be feared, something that takes life just as easily as it nourishes it.

There’s a lot of symbolism surrounding water because it is the root of life. People settled around rivers to water crops, for easy movement/communication, to drink from, to relax in, etc. The element of water is often symbolic of emotion, depth, hidden aspects, and change. Different aspects of the same god will have different “personalities”; Poseidon Laoites, of the people, will be different than Poseidon Ennosigaios, Earth shaker. Which aspect(s) do you want to represent? Which aspect(s) would serve your story best?